Capacitive measuring instruments for measuring fill levels have been known for many years. A probe protruding into the container and the container wall, or two sensors protruding into the container, form a capacitor. This capacitance C1 is dependent on the fill level and on the dielectric constant of the medium to be measured. Thus from the capacitance, a conclusion can be drawn as to the fill level.
One possible way of measuring this capacitance C1 is known as apparent current measurement. For instance, the amount of the alternating current flowing through the capacitor, made up of the sensor, the medium, and the container wall, at a certain frequency and voltage and at the capacitance C1 to be determined is measured by means of a conventional rectifier circuit. The apparent current Is, however, is dependent not only on the capacitance C1 but also on the conductivity R1 of the medium to be measured. Since especially with bulk goods, the conductivity R2 depends on various factors, such as temperature or humidity, the results obtained are imprecise.
One method for suppressing the influence of this parallel conductivity is measurement at relatively high frequencies. The proportion of the apparent current Is flowing through the capacitance C1 is proportional to the frequency, while conversely the proportion caused by the conductivity R1 remains constant. Thus at high frequencies, the capacitive proportion practically always predominates. However, experience teaches that with long probes with high parasitic inductances, measurement at high frequencies (>100 kHz) is difficult.
Another method of measuring the capacitance C1 is to measure not the apparent current Is but rather the blind current at a phase displacement angle of 90° between the current and the voltage, which is equivalent to measuring purely capacitance. This can be done for instance with the aid of a synchronous rectifier circuit (see German Patent DE 42 44 739 C2). For some media, this method has disadvantages. In media with a low dielectric constant and high conductivity, which can be measured well using apparent current measurement, difficulties arise because of the practically vanishing blind current. Moreover, such conventional synchronous rectifier circuits are known from experience to be vulnerable to electromagnetic interference.
Problems in measurement occur for instance from tolerances in the components used and from encrustation that can occur on the measurement probe from the medium to be measured. This encrustation sometimes severely affects the measurement signal and thus also the measured value.